This invention relates in general to electrophotography and, in particular, to an electrophotographic imaging member.
In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate photogenerating and charge transport layers. The photogenerating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, a charge transport layer and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor may also comprise additional layers such as an anti-curl back coating and an optional overcoating layer.
When one or more photoconductive layers are applied to a flexible supporting substrate, it has been found that the resulting photoconductive member tends to curl. Coatings may be applied to the side of the supporting substrate opposite the photoconductive layer to counteract the tendency to curl. However, difficulties have been encountered with these anti-curl coatings. For example, photoreceptor curl can sometimes still be encountered in as few as 1,500 imaging cycles under the stressful conditions of high temperature and high humidity. Further it has been found that during cycling of the photoconductive imaging member in electrophotographic imaging systems, the relatively rapid wear of the anti-curl coating also results in the curling of the photoconductive imaging member. In some tests, the anti-curl coating was completely removed in 150,000 to 200,000 cycles. This wear problem is even more pronounced when photoconductive imaging members in the form of webs or belts are supported in part by stationary guide surfaces which cause the anti-curl layer to wear away very rapidly and produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance. Also, the anti-curl coatings occasionally separate from the substrate during extended cycling and render the photoconductive imaging member unacceptable for forming quality images. It has also been found that when long webs of a flexible photoconductor having an anti-curl coating on one side of a supporting substrate and a photoconductive layer on the opposite side of the substrate are rolled into large rolls, the high surface contact friction of the anti-curl coating against the surface of the photoconductive imaging member causes dimples and creases to form on the photoconductive layer which result in print defects in the final developed images. Further, when the webs are formed into belts, the outer surface of the anti-curl belt experiences high surface contact friction with itself during shipment or storage at elevated temperatures which also causes creases and dimples to form which are seen as undesirable aberrations in the final printed images. Expensive and elaborate packaging is necessary to prevent the anti-curl coating from contacting itself. Further, difficulties have been encountered in continuous coating machines during the winter manufacturing of the coated photoconductive imaging members because of occasional seizing which prevents transport of the coated web through the machine for downstream processing.
Anti-curl layers will also occasionally delaminate due to poor adhesion to the supporting substrate. Moreover, in electrostatographic imaging systems, where transparency of the substrate and anti-curl layer is necessary for rear exposure to activating electromagnetic radiation, any exposure to activating electromagnetic radiation or any reduction of transparency due to opacity of the supporting substrate or anti-curl layer will cause a reduction in performance of the photoconductive imaging member. Although the reduction in transparency may in some cases be compensated for by increasing the intensity of the electromagnetic radiation, such increase is generally undesirable due to the amount of heat generated, as well as the greater cost to achieve higher intensity. An anti-curl layer which exhibits the above deficiencies is highly undesirable.
U.S. Pat. No. 4,654,284 to Yu et al discloses an anti-curl layer comprising a reaction product of a binder bifunctional coupling agent and crystalline particles such as silica. Although these inorganic particles provide wear resistance and reduced surface contact friction, they have exhibited one shortcoming of causing a sonic horn to wear when ultrasonically lap-joining the photoconductive imaging members into belts.
U.S. Pat. No. 4,209,584 to Joseph discloses photographic elements having anti-curl and anti-static layers. The anti-curl layer comprises a hydrophillic colloid which is hardened by a hardening agent. The anti-curl layer is not subject to wear since it is overcoated with the anti-static layer.
U.S. Pat. No. 4,520,089 to Tazuki et al discloses electrophotographic offset masters which are provided with a back coat layer. The back coat layer contains an inorganic pigment component such as sericite.
There continues to be a need to increase the durability and extend the life of the exposed anti-curl layer in an imaging device as well as to reduce frictional contact between members of the imaging device while maintaining electrical and mechanical integrity.